The present invention relates to a digital communication apparatus.
In the communication field, a spread-spectrum communication technique is suitable for a high-speed data transmission in the environment where the channel characteristics such as multipath fading undergo a considerable dynamic change.
Typical examples of spread-spectrum communication technique include a direct spread (DS) system and a frequency hopping (FH) system. The DS system is advantageous in view of small circuit size and high-speed data transmission, while the FH system is advantageous in view of channel capacity and communication reliability. Examples of the FH system include a high-speed FH system and a low-speed FH system. The high-speed FH system in which communication is made while the carrier frequency is being switched in a short period of time, is considerably increased in hardware size as compared with the low-speed FH system, but is advantageous in view of reliability against multipath fading.
Examples of a primary modulation in the FH system include a frequency shift keying (FSK) modulation, a phase shift keying (PSK) modulation and the like. In view of simplicity in circuit configuration requiring no phase control, the FSK modulation is relatively often used.
According to an arrangement-of an FH digital communication apparatus of prior art, the transmission throughput per channel, even for one-channel communication, is the same as that in communication using a plurality of channels.
According to another arrangement of the FH digital communication apparatus of prior art, carrier frequency waveforms are synthesized by a PLL synthesizer in the transmitter. This makes it difficult to switch the carrier frequency at a high speed of the order of micro second. Thus, such an arrangement is not suitable for the high-speed FH system. Further, the receiver requires, at its envelop line detector unit, analog band-pass filters having sharp amplitude characteristics in number equal to the number of carrier frequencies. This results in an increase in hardware. To achieve the high-speed FH system, it would be proposed that both the generation of waveforms and the detection of frequencies are conducted by a digital signal process. However, this disadvantageously excessively increases the frequency of a sampling clock for a digital signal process. On the other hand, when detecting frequencies using a discrete Fourier transform (DFT), it is required that the DFT operation interval is accurately in synchronism with the time slot. This has hitherto been difficult.
There is known a digital communication apparatus using a code multiplexing MFSK modulation using M carrier frequencies, M being an integer not less than 4. According to D. J. Goodman et al., "Frequency-Hopped Multilevel FSK for Mobile Radio", Bell System Technical Journal, Vol. 59, No. 7, pp. 1257-1275, September 1980, M frequencies (tones) are prepared in a predetermined band according to the high-speed FH system, and a unique code is assigned to each user on a time-frequency matrix. However, a high sampling rate is required in the DFT process, making it practically difficult to achieve the hardware.
There is now considered a digital communication apparatus of the mode changeover type arranged to make a frequency multiplex communication with either the MFSK or FH mode selected according to multiplicity. However, when the transmitter is not provided with a data scrambling function and the appearance probability of transmission data is uneven, the spectra of a transmission signal are also uneven. Further, when specific frequency components appear continuously, timing extraction becomes difficult in the receiver. This lengthens the time required for pulling into synchronism. Further, in the receiver, there are instances where, in an operation mode according to the MFSK mode, a plurality of reception signals are detected under the influence of noise, a spurious response or the like. In such a case, the maximum likelihood word cannot be determined. Further, in an operation mode according to the FH mode, too, when a plurality of words are calculated by a majority judgment, the maximum likelihood word can neither be determined.
In G. Einarsson, "Address Assignment for a Time-Frequency-Coded, Spread-Spectrum System", Bell System Technical Journal, Vol. 59, No. 7, pp 1241-1255, September 1980, two methods are proposed for generating hopping codes from data in a digital FH-MFSK communication system. One is based on the premise of a synchronous system, while the other is based on the premise of an asynchronous system (a code multiplexing system providing a chip synchronism between users, but not providing a frame synchronism between users). Both methods are based on a Reed-Solomon code. However, under the influence of frequency-selective fading, there might occur a miss detection (deletion) of all specific frequency components.